[OpaquePtr] Make GEPs work with opaque pointers

No verifier changes needed, the verifier currently doesn't check that
the pointer operand's pointee type matches the GEP type. There is a
similar check in GetElementPtrInst::Create() though.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D102744

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4168,7 +4168,8 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
       if (!Ty) {
         std::tie(FullTy, Ty) =
             getPointerElementTypes(FullBaseTy->getScalarType());
-      } else if (Ty != getPointerElementFlatType(FullBaseTy->getScalarType()))
+      } else if (!cast<PointerType>(FullBaseTy->getScalarType())
+                      ->isOpaqueOrPointeeTypeMatches(Ty))
         return error(
             "Explicit gep type does not match pointee type of pointer operand");